JP-2004-333133A discloses an inertial sensor having a device layer where a movable portion is formed. The device layer is sandwiched between upper and lower substrates so that the movable portion of the device layer can be sealed by the upper and lower substrates. The upper substrate serves as a cap for covering the movable portion and prevents entry of a foreign matter such as water into the movable portion.
The upper substrate has a through hole where the device layer is exposed. A pad formed in the device layer is exposed through the through hole, and a bonding wire is directly connected to the exposed pad so that the device layer can be electrically connected to external circuitry.
Such an inertial sensor is manufactured as follows. Firstly, the device layer and the lower substrate are joined together, and the movable portion is formed in the device layer. Then, the upper substrate having the through hole is joined to the device layer in such a manner that the through hole of the upper substrate can be aligned with the pad of the device layer. Then, the bonding wire is connected to the pad that is exposed through the through hole.
The through hole of the upper substrate is sized to be large enough to prevent the bonding wire from touching the upper substrate. Therefore, there is a problem that a chip size becomes large due to the large through hole. Further, there is another problem that a surface area of the upper substrate becomes small due to the large through hole. Accordingly, strength of the upper substrate is reduced, and there arises a difficulty in handling such as holding and carrying the upper substrate.